Fabrication of cr-y sheet



United States Patent 3,162,530 FABRECATIUN 6F Cr-Y SHEET Robert W.Harrison, Portland, Conn, assigns-r to United Aircraft Corporation, EastHartford, Conn., a corporation of Deiaware No Drawing. Fiied Feb. 12,1963, er. No. 257,840 9 Qiaims. (Cl. 75214) This invention relates tohigh-temperature, oxidationresistant alloys and more specifically to achromiumyttrium alloy and an improved method for making the same.

Prior to this invention chromium-yttrium was known to be a goodhigh-temperature, oxidation-resistant alloy. Since the maximumsolubility of yttrium in chromium is not much over 1%, the alloy mostcommonly used was Cr-l w/o Y (1 percent Y by weightbalance chromium).This alloy, as heretofore prepared by melting techniques, wassufliciently ductile to permit the formation of structural shapes byusual metal working methods but was not ductile enough to permit theformation of thin sheets and would not permit the formation of parts bystamping. Further, this alloy as previously prepared by powdermetallurgy fell far short of having the maximum possible theoreticaldensity of 7.16 g./cc.

Chromium-1 W/o Y is very susceptible to crack propagation. Suchmaterials are more easily fabricated when the grain size is small andpercent of theoretical density is at the maximum. Arm melting leads tocompletely dense material; however, the resulting grain size is verylarge (greater than ASTM 1). Powder metallurgy leads to a fine grainsize but heretofore maximum density Crw/o Y was unattainable.

It is therefore an object of this invention to produce a Cr-l w/o Yalloy having the maximum theoretical density by the powder metallurgyapproach with a fine grain zize (less than ASTM 7).

Nitrogen contamination has been shown by previous investigations to bevery detrimental to the ductility of Cr-l w/o Y by raising thebrittle-to-ductile transformation temperature. By keeping the nitrogencontent as low as possible, preferably under 100 p.p.m. (parts permillion), a ductile material can be produced that can be rolled tosheet. It is therefore another object of this invention to provide aprocess by which high purity can be maintained from the starting powdersto the finished product, namely 8 mil Cr-l w/o Y sheet.

Due to its high vapor pressure, chromium and its alloys, such as Cr-lw/o Y, cannot be sintered at high temperatures and high vacuum (Xl0 mm.Hg). Sintering in pure hydrogen at high temperatures is a bettertechnique for pure chromium; however, this technique is not applicableto the Cr-l w/o Y alioy since the yttrium will hydride. Previousinvestigations employing a onestep high temperature sintering treatmentfor the densification were unsuccessful in achieving maximum density.

It is therefore a principal object of this invention to provide atwo-step sintering process to achieve mam'murn density pure Cr-l w/o Ycompacts for rolling to 8 mil stampable sheet. I

In the first step the yttrium is put into solution in the In making theimproved alloy, high purity chromium powder (nitrogen less than 20p.p.m.), either -200 mesh Lunex or 270 Cooper, isblended with 325Michigan Chemical yttrium powder. This blending is done under argon for20 hours. Compacts, 1" diameter X 3 length, are made without exposure toair by pressing at 50 t.s.i. in a sealed die. Exposure to air is avoidedby providing an argon atmosphere in the die. Sintering is next performedand, in accordance with this invention, it is performed in two distinctsteps.

In the first step the compact is heated to 2700 F. in a vacum of 5 1()mm. of Hg for /2 hour. Heating is then continued for 1 /2 hours in aF/satmosphere of pure hydrogen which can be made from the decomposition ofyttrium hydride. By heating the compact in vacuum prior to introducingthe hydrogen, yttrium will go into solution without the formation ofyttrium hydride.

ensities at this point in the, process measure 85% of theoretical. Coldcoining at t.s.i. brings the density of the compact up to 93%oftheoretical.

In the second sintering step the coined compact is heated for 5 hours inpure hydrogen at 2700 F. Following this second step the density rangesfrom 98l00% of theoretical density. Nitrogen content at this point isless than 40 p.p.m.

Dense compacts are then sealed in evacuated pure nickel cans at 5 l0 mm.of Hg. By press forging at 400 t.s.i. at 2350 F., a A" thick asembly isobtained having a cross section of nickel-Cr-l w/o nickel. The assemblyis next hot rolled at 2200 F. to 36 mils and then further rolled at1800* F. to 9 mils. The nickel sheet is then removed by pickling innitric acid. Nitrogen content of the Cr-l w/o Y at this point is lessthan 80 p.p.m. As well as protecting the Cr-l w/o Y from contaminationduring rolling, the use of pure nickel as a sheath .material gives abetter surface finish to the Crl w/o Y sheet than other lessoxidation-resistant materials and still allows removal by picklinginnitric acid which will not attack the Cr-l w/o Y sheet. To furtherimprove the appearance of the Cr-l w/o Ysheet, an electrolytic polish inhydrochloric acid can be used.

In practicing this invention, I use this final step of subjectingtheCr-l w/o Y sheet to a final pickling in hydrochloric acid to removeabout 1 mil of the sheet following theremoval of the nickel can becauseit considerably improves the ductility of the Cr-l w/o Y. This step isparticularly recommendedif stamping operations are to be subsequentlyperformed on the sheet. I do not know the reason for this increase inductility and stampabiiity but it is quite pronounced and cannotbeattributed to the mere reduction in thickness of the sheet. Attemptsto roll Cr-l w/o Y without the nickel sheath have not,

- however, been'successful.

chromium and sintering is initiated. The compact is a Before removal ofthe nickel sheath-the 8 mil sheet can be .cu t'with a shears withoutcracking.-- Also, for the 800 F. Testing has shown that sheet materialformed by this process retains maximum ductility down to 500 F. It willthus be evident that by this invention a new process has been providedfor producing Cr-l w/o Y sheet and that the sheet produced has not onlythe maximum theoretical densitybut also has such improved ductility. asto permit 8 mil stampab-le sheet to be rolled from it.

What 'it is desired to claim is:

1. A binary sintered alloy of chromium-yttrium consisting of Cr-1 w/o Yhaving less than 100 ppm. nitrogen and having a density of approximately100% of theoretical maximum density and having extreme ductilityenabling sheath rolling into a stampable 8 mil sheet.

2. A stampable sheet of sintered chromium-yttrium alloy consisting ofCr-l w/o Y having less than 100 p.p.m. nitrogen and having a densityapproaching the maximum theoretical density of 7.16 g./cc. and having agrain size less than ASTM 7.

3. A ductile sintered alloy of chromium-yttrium consisting of Cr-l w/o Yhaving less than 100 ppm. nitrogen and having a density of approximately7.16 g./cc. and a grain size less than ASTM 7.

4. A method for producing a dense and ductile Cr-l w/o Y sheet by powdermetallurgy which comprises, blending under argon fine chromium andyttrium powders having nitrogen less than 20 p.p.m., pressing theblended powders into a compact, putting the yttrium into solution in thechromium without yttrium hydride formation by heating the compact to2700 F. in a vacuum of 5X10- mm. of Hg for /2 hour, then introducing /3atmosphere of pure hydrogen and continuing heating for 1 /2 hours, coldcoining the compact at 60 t.s.i., sintering for 5 hours in pure hydrogenat 2700 F., sealing the compact in a nickel can at 5 10 mm. of Hg, pressforging the assembly at 400 t.s.i. at 2350 F., hot rolling the assemblyat 2200 F. to approximately 36 mils, continuing rolling at 1800 F. toapproximately 9 mils, and pickling in nitric acid to remove the nickelsheath.

5. A method for producing a dense and ductile Cr-l w/o Y sheet by powdermetallurgy which comprises, blending under argon fine chromium andyttrium powders having nitrogen less than 20 p.p.m., pressing theblended powders into a compact, putting the yttrium into solution in thechromium without yttrium hydride formation by heating the compact to2700 F. in a vacuum of 5X10 mm. of Hg for /2 hour, then introducing /(aatmosphere of pure hydrogen and continuing heating for 1 /2 hours, coldcoining the compact at 60 t.s.i., sintering the compact for 5 hours inpure hydrogen at 2700 F., sealing the compact in a nickel can at 5 10mm. of Hg, press forging the assembly at 400 t.s.i. at 2350 F., hotrolling the assembly at 2200 F. to approximately 36 mils, continuingrolling at 1800 F. to approximately 9 mils, removing the nickel sheathby pickling in nitric acid, and pickling the Cr-1 w/o Y sheet inhydrochloric acid to remove a small amount of the surface of the sheet.

6. A method for producing an 8 mil Cr-l w/o Y sheet by powder metallurgywhich comprises, blending under an inert gas chromium and yttriumpowders containing less than 100 p.p.m. of nitrogen, pressing thepowders into a compact, putting the yttrium into solution in thechromium without yttrium hydride formation by heating the compact to2700 F. in a vacuum of 5 10 mm. of Hg for /2 hour, introducing /3atmosphere of pure hy drogen and continuing heating for 1 /2 hours, andcold coining the compact at 60 t.s.i., sintering for 5 hours in purehydrogen at 2700 F., sealing the compact in a nickel can at 5 10- mm. ofHg, press forging the assembly at 400 t.s.i. at 2350 F., hot rolling theassembly to finished dimensions, and removing the nickel sheath bypickling in nitric acid.

7. A sintering process for converting a Cr-1 w/o Y compact of chromiumand yttrium powders into a compact having a density ranging from 98-100%of maximum theoretical density, first putting the yttrium into solutionin the chromium Without yttrium hydride formation by heating the compactto 2700 F. in a vacuum of 10 mm. of Hg for /2 hour, introducing /aatmosphere of pure hydrogen while continuing the heating for 1 /2 hours,cold coining at 60 t.s.i. to bring the density up to about 93% ofmaximum theoretical density, and then heating the coined compact for 5hours in pure hydrogen at 2700 F to bring the final density of thecompact into the range of 98100% of theoreticaldensity.

8. A method for producing by powder metallurgy a stampable Cr-l w/ o Ysheet having approximately 100% maximum theoretical density whichcomprises,- blending fine chromium and yttrium powders having nitrogenless than 20 ppm. under argon gas for 20 hours, pressing the blendedpowders into a compact in a die in an argon atmosphere at 50 t.s.i.,heating the compact to 2700 F. in a vacuum of 5 mm. of Hg for /2 hour,introducing /3 atmosphere of pure hydrogen and continuing heating for 1/2 hours, cold coining the compact at 60 t.s.i., heating the compact for5 hours in pure hydrogen at 2700 F., sealing the compacts in nickel cansat 5 1O- mm. of Hg, press forging at 400 t.s.i. at 2350 F., hot rollingthe assembly at 2200 F. to 36 mils, continuing rolling at 1800 F. toapproximately 9 mils, and pickling in nitric acid to remove the nickelsheath.

9. A method for producing by powder metallurgy a stampable Cr-l w/o Ysheet having approximately 100% maximum theoretical density whichcomprises, blending fine chromium and yttrium powders having nitrogenless than ppm. under argon gas for 20 hours, pressing the blendedpowders into a compact in a die in an argon atmosphere at t.s.i.,heating the compact to 2700 F. in a vacuum of 5 10- mm. of Hg. for /2hour, introducing /3 atmosphere of pure hydrogen and continuing heatingfor 1 /2 hours, cold coining the compact at t.s.i., heating the compactfor 5 hours in pure hydrogen at 2700 F., sealing the compacts in nickelcans at 5 10 mm. of Hg, press forging at 400 t.s.i. at 2350 F., hotrolling the assembly at 2200" F. to 36 mils, continuing rolling at 1800F. to approximately 9 mils, pickling in nitric acid to remove the nickelsheath, and pickling the Cr-l w/o Y sheet in hydrochloric acid to removea small amount of the surface of the sheet to further enhance itsductility.

References Cited by the Examiner UNITED STATES PATENTS 2,206,395 7/40Gertler 214 3,015,559 l/62 McGurty 7584 3,085,007 4/63 Aronin 29182OTHER REFERENCES Atomic Energy Commission Document APEX-680,Chromium-Base Alloy Development, April 1962, pages 13, 34 and 37.

CARL D. QUARFORTH, Primary Examiner.

REUBEN EPSTEIN, Examiner.

1. A BINARY SINTERED ALLOY OF CHROMIUM-YTTRIUM CONSISTING OF CR-1 W/O YHAVING LESS THAN 100 P.P.M. NITROGEN AND HAVING A DENSITY OFAPPROXIMATELY 100% OF THEORETICAL MAXIMUM DENSITY AND HAVING EXTREMEDUCTILITY ENABLING SHEATH ROLLING INTO A STAMPABLE 8 MIL SHEET.
 3. ADUCTILE SINTERED ALLOY OF CHROMIUM-YTTRIUM CONSISTING OF CR-1 W/O YHAVING LESS THAN 100 P.P.M. NITROGEN AND HAVING A DENSITY OFAPPROXIMATELY 7.16 G./CC. AND A GRAIN SIZE LESS THAN ASTM 7.